unusual warnings in gcc 16 with LTO

The crux of the warning is:

/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1209:11: warning: writing 2 bytes into a region of size 0 [-Wstringop-overflow=]
 1209 |     memcpy(out, digits2(value), 2);
      |           ^
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h: In function ‘write_fixed’:
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2433:8: note: at offset [-11, -1] into destination object ‘buffer’ of size 11
 2433 |   Char buffer[digits10<UInt>() + 2];
      |        ^

Note this only happens since

$ gcc --version
gcc (GCC) 16.1.1 20260430

which has just been pushed into the repos on Arch Linux.

compiling fmtlib as static lib, and then linking with LTO enabled...

set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)

FetchContent_Declare(
  fmt
  GIT_REPOSITORY https://github.com/fmtlib/fmt.git
  GIT_TAG 12.1.0
)
FetchContent_MakeAvailable(fmt)

add_executable(app ...)
target_compile_features(app PRIVATE cxx_std_26)
target_link_libraries(app PRIVATE fmt )

and during link stage, I get these odd warnings

cmake -GNinja -S . -B ./build/gcc/release -DCMAKE_COLOR_DIAGNOSTICS=ON -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -DCMAKE_BUILD_TYPE=release
....
[2/11] /usr/bin/g++  -I/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include -O3 -DNDEBUG -std=c++20 -flto=auto -fno-fat-lto-objects -fdiagnostics-color=always -MD -MT _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o -MF _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o.d -o _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o -c /home/oliver/psmem/build/gcc/release/_deps/fmt-src/src/os.cc
[3/11] /usr/bin/g++  -I/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include -I/home/oliver/psmem/include -I/home/oliver/psmem/build/gcc/release/_deps/termbox2_src-src -I/home/oliver/psmem/build/gcc/release/_deps/unordered_dense-src/include -O3 -DNDEBUG -std=c++26 -flto=auto -fno-fat-lto-objects -fdiagnostics-color=always -Wall -Wextra -Wshadow -fdiagnostics-color=always -MD -MT CMakeFiles/psmem.dir/src/proc.cpp.o -MF CMakeFiles/psmem.dir/src/proc.cpp.o.d -o CMakeFiles/psmem.dir/src/proc.cpp.o -c /home/oliver/psmem/src/proc.cpp
[4/11] /usr/bin/g++  -I/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include -O3 -DNDEBUG -std=c++20 -flto=auto -fno-fat-lto-objects -fdiagnostics-color=always -MD -MT _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o -MF _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o.d -o _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o -c /home/oliver/psmem/build/gcc/release/_deps/fmt-src/src/format.cc
[5/11] : && /usr/bin/cmake -E rm -f _deps/fmt-build/libfmt.a && "/usr/bin/gcc-ar" qc _deps/fmt-build/libfmt.a  _deps/fmt-build/CMakeFiles/fmt.dir/src/format.cc.o _deps/fmt-build/CMakeFiles/fmt.dir/src/os.cc.o && "/usr/bin/gcc-ranlib" _deps/fmt-build/libfmt.a && :
...
[11/11] : && /usr/bin/g++ -O3 -DNDEBUG -flto=auto -fno-fat-lto-objects -fuse-ld=mold -Wl,--dependency-file=CMakeFiles/psmem.dir/link.d CMakeFiles/psmem.dir/app/psmem.cpp.o CMakeFiles/psmem.dir/src/proc.cpp.o -o psmem  _deps/fmt-build/libfmt.a  libtermbox2.a  _deps/fmt-build/libfmt.a && :
In function ‘write2digits’,
    inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2415:17,
    inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2407:13,
    inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2434:31,
    inlined from ‘write_significand’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2461:20,
    inlined from ‘operator()’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2539:33,
    inlined from ‘write_padded’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1701:9,
    inlined from ‘write_padded’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1710:43,
    inlined from ‘write_fixed’ at /home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2536:44:
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:1209:11: warning: writing 2 bytes into a region of size 0 [-Wstringop-overflow=]
 1209 |     memcpy(out, digits2(value), 2);
      |           ^
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h: In function ‘write_fixed’:
/home/oliver/psmem/build/gcc/release/_deps/fmt-src/include/fmt/format.h:2433:8: note: at offset [-11, -1] into destination object ‘buffer’ of size 11
 2433 |   Char buffer[digits10<UInt>() + 2];
      |        ^

Things I have tried:

• disabling LTO removes the warnings
• the above uses the mold linker but normal ld produces the same warnings
• changing to -DCMAKE_BUILD_TYPE=debug makes the warnings go away.
• using latest git fmt instead of TAG=12.1.0 produces the same warnings (now line 1208)

These may very well be over eager new gcc....or bizarre UB in my code?